Manufacturers of implantable medical devices have long attempted to understand, and in turn improve, the performance of materials used in blood-contacting applications (Leonard, E. F., et al. Ann. N.Y. Acad. Sci. 516, New York, Acad. Sci., New York, 1987). The biological response of the body, as well as problems with infection, have hindered the application of implantable, disposable, and extracorporeal devices. Anticoagulant drugs, such as heparin and coumadin, can improve the use of such devices, although anticoagulants have their own corresponding risks and drawbacks. For these reasons, development of materials having greater compatibility with blood has been pursued aggressively (Sevastianov, V. I., CRC Crit. Rev. Biocomp. 4:109, 1988).
Two general strategies that have been used to develop improved blood-contacting materials include modifying the chemistry of the bulk material itself, and/or modifying the interfacial properties of the material. With regard to the latter approach, several classes of materials have been covalently bonded onto blood-contacting surfaces with the goal of improving blood compatibility. These include anticoagulants, such as heparin and hirudin; hydrogels; polyethylene oxide (PEO); albumin binding agents; cell membrane components; prostaglandins; and sulfonated polymers. These approaches have met with varying degrees of success in terms of reducing protein adsorption, platelet adhesion and activation, and thrombus formation. Unfortunately, no approach has yet been shown to be universally applicable for improving blood-biomaterial interactions.
As mentioned above, albumin binding agents have been considered for use on biomaterials. Biomaterials having a high surface concentration of albumin have been shown to be less likely to initiate the fibrin cascade and platelet attachment than those having a high concentration of other serum proteins, such as fibrinogen, fibronectin, or immunoglobulins. On many polymeric materials, however, fibrinogen is often the predominant protein adsorbed from protein mixtures or plasma. For these reasons, investigators have attempted to immobilize albumin onto materials or to design biomaterial surfaces that will enhance binding of endogenous albumin from blood, thus mitigating the adsorption of fibrinogen and consequent thrombogenic phenomena.
In this respect, a number of different approaches have been employed to date. These approaches include passive adsorption or covalent immobilization of albumin to the surface, and the development of surfaces designed to selectively bind endogenous albumin from circulating blood, the latter using alkyl chain-modified materials and other means.
Munro, et al., U.S. Pat. No. 4,530,974, discloses a method of adsorbing albumin to a water-insoluble polymer such as polyurethane by covalently binding to the surface a nonionic hydrophobic aliphatic chain to which serum albumin will selectively bind.
Frautschi et al., U.S. Pat. Nos. 5,017,670 and 5,098,977, teach methods for covalent attachment of aliphatic extensions of 12 to 22 carbon atoms to water-insoluble polymers containing aromatic rings and ring structures with adjacent secondary hydroxyls for increased albumin binding.
Eaton, U.S. Pat. No. 5,073,171, describes a biocompatible prosthetic device incorporating an amount of an albumin binding dye effective to form a coating of endogeneous albumin on the device when the device is in contact with a physiological fluid containing albumin.
While some or all of these various strategies can be used to enhance the binding of endogenous albumin to blood-contacting material surfaces, and in turn to reduce fibrinogen binding, these approaches are each limited in one or more respects. Alkyl chain-modified surfaces have been shown to increase albumin binding and decrease fibrinogen binding, but these effects were fairly limited, for instance, to a short term time frame (generally less than one hour). In addition, various other surface modification methods discussed above are useful for only a narrow range of substrate materials.
On another subject, the assignee of this application has developed the ability to attach bioactive groups to a surface by covalently bonding those groups, directly or indirectly, to the surface. For instance, U.S. Pat. Nos. 4,722,906, 4,979,959, 4,973,493 and 5,263,992 relate to devices having biocompatible agents covalently bound via photoreactive groups and a chemical linking moiety to the biomaterial surface. U.S. Pat. Nos. 5,258,041 and 5,217,492 relate to the attachment of biomolecules to a surface through the use of long chain chemical spacers. U.S. Pat. Nos. 5,002,582 and 5,512,329 relate to the preparation and use of polymeric surfaces, wherein polymeric agents providing desirable properties are covalently bound via a photoreactive moiety to the surface. In particular, the polymers themselves exhibit the desired characteristics, and in the preferred embodiment, are substantially free of other (e.g., bioactive) groups.
It would be highly desirable to be able to attach albumin to a biomaterial surface in a manner that is suitably stable for extended use, particularly in a manner that permits the albumin to be replenished over time and in the course of use.